1. Field of the Invention
The present invention relates to glass compositions suitable for use in stabilization of radioactive, hazardous, or mixed waste, having relatively low melting points and relatively high amounts of lithia (Li2O), to methods of making these glass compositions, and to methods for using the compositions to immobilize waste materials.
2. Description of the Related Art
Various hazardous, radioactive, and mixed (both hazardous and radioactive) wastes, including heavy metal wastes such as lead paint and contaminated soils, require stabilization in solid forms that meet regulatory disposal criteria promulgated by government agencies like EPA and NRC. As discussed below, these wastes originate from a variety of sources, and consequently can exist in a variety of forms, including aqueous waste streams, sludge solids, mixtures of aqueous supernate and sludge solids, combinations of spent filter aids from waste water treatment and waste sludges, supernate alone, incinerator ash, incinerator offgas blowdown, or combinations thereof, geological mine tailings and sludges, asbestos, inorganic filter media, cement waste forms in need of remediation, spent or partially spent ion exchange resins or zeolites, contaminated soils, lead paint, etc.
Many industrial processes generate hazardous wastes in the form of aqueous waste streams, sludge solids, aqueous supernate, incinerator ash, incinerator off gas condensate, and so forth. Waste treatment processes may themselves generate secondary hazardous wastes. For example, solids can be filtered from an aqueous waste stream by passing the stream through filter aids, such as perlite (PERFLO) or diatomaceous earth filters. The spent filter medium is impregnated with the materials removed from the waste stream, such as heavy metals and other hazardous or radioactive substances. The spent filtration wastes must themselves be treated and stabilized before disposal. As used herein, the term xe2x80x9chazardous wastexe2x80x9d includes wastes containing substances commonly recognized as hazardous, including but not limited to, chemical wastes, radioactive wastes, mixed chemical and radioactive wastes, heavy-metal-containing wastes, and hazardous organics.
Stabilizing hazardous wastes using currently available technology is expensive and requires enormous resources of equipment and personnel. Stabilization processes must be operated within guidelines established under the Resource Conservation and Recovery Act (RCRA), and the stabilized product must meet stringent state and federal standards. In the case of radioactive or mixed wastes, the stabilized wastes must often be stored for long periods of time waiting for decay of the radioactive components before transportation to an approved underground repository. Minimizing the waste volume is important in minimizing storage, transportation, and final disposal costs.
Incinerators are often used to destroy the hazardous constituents of solid and liquid wastes, as well as municipal garbage. Byproducts of incineration include bottom ash, aqueous incinerator offgas condensate (blowdown), or mixtures of ash and offgas condensate, all of which may contain residual hazardous and/or radioactive substances.
Radioactive waste may be further categorized into high level waste (HLW) and low level waste (LLW). High level waste is generally generated by reprocessing of spent nuclear fuel and other irradiated material, weapons production, research and development, etc. High level waste resulting from fuel reprocessing generally is an acidic, highly radioactive, and heat producing liquid that is generally either calcined to a dry, granular solid or neutralized, dehydrated, and stored as a damp salt, sludge, and supernate liquid. Low level waste generally contains more radioactivity than is allowed for municipal disposal, but are not sufficiently radioactive to produce substantial amounts of heat Low level waste typically includes contaminated soil, clothing, gloves, resins, waste sludges, etc.
Hazardous wastes may be solidified by vitrification (incorporation into a glass matrix) or cementation. In typical cementation processes, cement-forming materials are added to the waste; any water in the waste solution remains in the solidified product. Therefore, the solidified product has a larger volume than the original waste solution. Also, water, including groundwater, can leach compounds out of cement over time and cement is naturally porous, so the cement-stabilized product must be stored in leak-proof containers to prevent leaching.
Glass is the most long-term environmentally acceptable waste form. Glass is stable and extremely durable. Moreover, the hazardous species are chemically bonded in the glass structure, forming a substantially nonleachable composition. A number of vitrification processes for hazardous wastes have been described. Wheeler (U.S. Pat. No. 4,820,325) stabilizes toxic waste using a glass precursor material such as diatomaceous earth mixed with a compatible glass precursor material such as soda ash, lime (CaO) and alumina. The normally leachable toxicant becomes fixed within the glass when the mixture is vitrified. Hayashi, et al. (U.S. Pat. No. 4,725,383) add ZnO, or a mixture of ZnO with Al2O3 and/or CaO, to a radioactive sodium borate waste solution. The resulting mixture is dehydrated and melted to produce a vitrified solid solution. Schulz, et al. (U.S. Pat. No. 4,020,004) vitrify radioactive ferrocyanide compounds by fusion with sodium carbonate (Na2CO3) and a mixture of basalt and B2O3, or silica (SiO2) and lime (CaO).
As discussed above, solidification of these and other wastes by glassification or vitrification is known, and generally involves combining glass forming compounds and/or natural rock, such as basalts or nepheline syenite, with the waste materials, and melting this mixture at temperatures sufficient to vitrify the mixture and immobilize the waste species in the resulting glass. The waste materials become dissolved in the melt and atomically bonded to the glass matrix that forms when the melt is cooled.
However, vitrification processes are not perfect, and problems are sometimes experienced due to the often limited solubility of the waste materials in the melt during glassification and/or due to the volatility of some or all of the waste species at the relatively high temperatures reached during the vitrification. Jantzen, xe2x80x9cSystems Approach to Nuclear Waste Glass Development,xe2x80x9d J. Non-Cryst. Solids, 84 (1-3), 215-225, 1986. As a result, a glass forming additive that lowered the melting temperature of the mixture of waste and glass formers, thereby decreasing the amount or likelihood of waste volatilization, would be very desirable. In addition, a glass forming additive that increased the solubility of waste materials in the glass forming mixture, thereby increasing the amount of waste atomically bonded in the glass, increasing the waste loading capacity of the glass, and decreasing the disposal volume, would also be very desirable.
Lithia (Li2O) has been disclosed to accelerate the dissolution of sand grains and increase melt rate. R. M. Wiker, Glass Industry, 37, 28, (1956). Lithia has also been disclosed to have a lower volatility than soda or potash at equal molar concentrations at 1400xc2x0 C. Volf, The Chemical Approach to Glass, 1984. Lithia has been disclosed to increase the viscosity of glass at low temperatures, N. W. Taylor et al., J. Amer. Ceram. Soc., 20, p. 296 (1937) and 24, p. 103 (1942), and to decrease the viscosity of glass at high temperatures, G. Heidtkamp et al., Glastechnick Bericht, 14, p. 99 (1936), as compared to soda (Na2O) and potash (K2O) containing glasses. Lithia is also disclosed to increase the modulus of elasticity in glasses compared to soda and potash. The mobile Li+ ion has also been disclosed to facilitate transmission of electric current, so that glasses containing Li+ melt at a faster rate in Joule heated melters than in gas fired commercial melters. Small amounts of Li+ have been disclosed to improve the electrical resistance during glass melting, decrease glass density (since Li+ is a light atomic weight clement), and impart a low coefficient of thermal expansion to the glass. Lithia is used widely in glass ceramics, such as CHEMCOR and CORNINGWARE (Corning). Volf, The Chemical Approach to Glass, 1984.
Despite all of this, lithia is not considered to be a conventional glass component, and is used only for highly specialized applications in the commercial glass industry. Lithia is used in amounts in the range of about 0.7 to about 1.5 wt % to improve meltability in sealing glasses, used, e.g., to seal tungsten metal to glass or to seal different types of glass together. Lithia is also used in amounts of about 0.25 wt % to improve meltability in glass for discharge tubes and large mirrors of astronomical telescopes. Volf, The Chemical Approach to Glass, 1984.
One of the reasons for the failure of the art to use lithia more extensively in glassmaking is that the production of commercial container glasses, e.g., commercial bottle glass, and the production of commercial window glass typically does not involve the use of glass forming additives to lower the melting temperature of the glass. The melting point of these glasses is not of great concern because there are no hazardous species to volatilize, and they are routinely formulated to melt at temperatures of around 1300xc2x0 C. to 1400xc2x0 C. Lithia in particular is typically avoided in the commercial glass industry because it can cause undesirable effects, such as phase separation. Phase separation results from inhomogeneities in the glass which form regions of liquid-liquid immiscibility at higher temperatures that are retained when the glass cools. Phase separation can result in regions that are quite small and dispersed throughout the glass, and phase separated regions can have different optical and durability properties. W. Vogel, Chemistry of Glass, pp. 69-95, American Ceramic Society, 1985. These factors can combine to make phase separated container or window glass undesirably opaque. Schott Guide to Glass. 
In addition, it is often considered undesirable to mix different alkali oxides in commercial glasses due to the xe2x80x9cmixed alkalixe2x80x9d effect, i.e., a significant reduction in the diffusion coefficient of the original alkali ion due to the presence of the second alkali ion, irrespective of the relative size of the alkali ions. M. Hara, Ion-channel match in mixed alkali glasses, J. Non-Cryst. Solids, 131 (1991). This effect can result in abnormal or nonlinear properties as a function of composition for a wide range of properties, including electrical conductivity of the melt, an important parameter in electric or Joule heated melters.
Jantzen, U.S. Pat. No. 5,102,439, disclose a borosilicate waste glass having about 5 wt % Li2O produced in a melter operating at 1160xc2x0 C., but does not suggest that the presence of lithia has any appreciable effect on the melting point of the glass. To the contrary, the nonbridging oxygen equations described by Jantzen do not indicate that lithia has any properties that differentiate it from other alkali metal oxides.
Lead based paint and other coatings, as discussed above, is a form of hazardous waste subject to EPA control and regulation. Kumar et al., U.S. Pat. No. 5,292,375, disclose a process for removal of lead based paint without generation of airborne particles of hazardous waste, and without the need to control lead-containing water solutions. In the process of Kumar et al., particles of a glass mixture are flame sprayed onto the coated surface to form an overlying layer of glass material. As this layer cools, it spalls and separates from the coated structure, taking at least some of the lead based coating with it. This can be repeated as necessary to remove the lead based coating from the underlying structure. The spalled glass fragments can then be collected, remelted, tested for leachability and other regulatory compliance, and disposed of in a landfill. In general, the glass fragments obtained by the Kumar et al. process pass EPA regulations, while the material generated by sandblasting lead based paint will not. The glasses disclosed by Kumar et al. did not contain lithia or ferric oxide, and Kumar et al. make no suggestion to use these compounds. However, it has been disclosed that the spray properties (fluidity) and decreased retention of lead in the structure can be enhanced by the presence of about 2 wt % lithia and about 12.3 wt % ferric oxide. Marra et al., Glass Composition Development for a Thermal Spray Vitrification Process, Ceramic Trans. vol. 72, pp. 419-426, Am. Cer. Soc. (1996).
It is an object of the present invention to provide a method for decreasing the melting point of glass compositions used to immobilize hazardous, radioactive, or mixed waste, so that hazardous or radioactive species are less likely to volatilize during the melting process.
It is another object of the present invention to provide a method for increasing the solubility of hazardous or radioactive species in glass compositions for immobilizing them, and to increase the waste loadings of waste glass immobilization systems.
It is another object of the present invention to provide a process suitable for disposal of radioactive and hazardous waste, such as waste sludges and supernates, spent filter aids, incinerator ash, mixtures of incinerator ash and incinerator blowdown, geological mine tailings, asbestos, loaded ion exchange resins, contaminated soils, and lead based paint and other coatings.
It is another object of the present invention to provide glass compositions that have significantly decreased melting temperatures as compared to those compositions generally used for waste stabilization, including borosilicate glass, soda-lime-silica glass, soda-baria-silica glass, and soda-magnesia-silica glass.
It is another object of the present invention to provide glass compositions whose preparation reduces the occurrence of melter off-gas line pluggage, allows the use of less expensive and more robust melter designs, and provides longer melter design life.
It is another object of the invention to provide waste glass production processes and compositions that lower vitrification temperatures, increase waste loadings, provide for large waste volume reductions, and produce durable glasses suitable for waste stabilization and capable of meeting regulatory guidelines.
These and other objects and advantages are achieved by the present invention, which is directed to processes for lowering the vitrification temperature of waste glass, to processes for vitrifying radioactive, hazardous, and mixed wastes using these lowered temperatures, and to glass compositions produced by these processes.
In one sense, the present invention involves the discovery that the addition of lithia or compounds that will form lithia under vitrification conditions, and in particular addition of lithia or lithia formers in place of a portion of soda, or potash, or soda or potash formers, as a glass former and/or fluxing agent will significantly lower the melting point, and hence the vitrification temperature, of waste glass mixtures. Because the glass melter can then be operated at a significantly lower temperature, it is possible to contain more of the hazardous and/or radioactive species in the waste material, rather than volatilizing these species. Addition of lithia or lithia formers also increases solubility of the species in the glass, which enhances the stabilization and retention of the species in the glass after cooling. This effect has been found to be useful in borosilicate glasses, soda-lime-silica glasses, soda-baria-silica glasses, and soda-magnesia-silica glasses, in the presence or absence of Substantial amounts of ferric oxide. The lithia may replace all or a portion of the alkali oxide glass formers typically used in these glass forming systems. This replacement may be on a mole percent basis, or on a weight percent basis.
Since clarity of the glass and mixed alkali effects, which are of concern for commercial glasses, are not of concern when stabilizing waste in glass, lithia concentrations of between about 0.16 wt % and 11 wt %, more particularly between about 0.16 wt % and about 9.30 wt %, based upon the total oxide glass formers in the glass composition, are used according to the present invention to lower the melt temperatures of, e.g., sodium borosilicate glasses, soda-lime-silica glasses, soda-baria-silica glasses, and soda-magnesia-silica glasses for a variety of wastes, including wastes found at Savannah River Site, Fernald, Oak Ridge, Rocky Flats, and United States Army facilities, Enhanced stabilization and retention of hazardous, radioactive, and mixed waste, including heavy metals, was achieved by adding lithium compounds as glass formers, usually as Li2CO3, which converts to Li2O in the glass when the glass formers and waste are reacted at elevated temperatures.
In one embodiment, the present invention is directed to a method of vitrifying radioactive, hazardous, or mixed waste having the steps of:
(1) mixing said waste with glass formers such that the resulting mixture comprises SiO2 and alkali oxide glass formers, wherein said alkali oxide glass formers comprise lithia formers and other alkali oxide glass formers in amounts such that the lithia formers, calculated as Li2O, are from 11.0 wt % to about 76 wt % of the total alkali oxide glass formers, calculated as M2O, where M is an alkali metal; and
(2) melting the resulting mixture at a temperature of between about 1050xc2x0 C. and about 1250xc2x0 C. and cooling to form a glass composition. This mixture may be suitable for forming either an alkali oxide borosilicate glass, or an alkali oxide-lime-silica glass. As used herein, the term alkali oxide borosilicate glass, alkali oxide-lime-silica glass, alkali oxide-baria-silica glass, or alkali oxide-magnesia-silica glass denotes a glass composition where the typical alkali oxide, sodium oxide, has been at least partially replaced or has been supplemented by lithia. Alkali oxide borosilicate and soda borosilicate glasses are those having a B2O3 content as defined in ASTM C162, i.e., 5 wt % or more B2O3, based upon the total weight of oxide glass formers in the glass composition (and excluding waste components that are not or do not become oxide glass formers during vitrification). The terms xe2x80x9calkali borosilicate,xe2x80x9d xe2x80x9calkali-lime-silica,xe2x80x9d xe2x80x9calkali-baria-silica,xe2x80x9d and xe2x80x9calkali-magnesia-silicaxe2x80x9d are synonymous with the xe2x80x9calkali oxidexe2x80x9d terms described above.
In another embodiment, the present invention is directed to a method of decreasing the melting point of a waste glass that contains sodium oxide, potassium oxide, rubidium oxide, cesium oxide, or combinations thereof and that immobilizes radioactive, hazardous, or mixed waste, having the steps of:
preparing a mixture of waste and glass formers, comprising lithia or a lithia former, selected from the group consisting of Li0 and lithium compounds that convert to lithia during melting at elevated temperatures, in an amount sufficient to provide between about 0.16 wt % and about 11 wt % Li2O in the glass composition, based upon the total weight of oxide glass formers in the glass composition; and
heating the mixture to a temperature below the melting point of the corresponding mixture without said lithia or lithia former.
In another embodiment, the present invention is directed to an alkali oxide borosilicate glass composition suitable for immobilizing low level radioactive, hazardous, or mixed waste, containing:
(a) SiO2 in an amount ranging from about 35 wt % to about 50 wt %;
(b) B2O3 in an amount ranging from about 5.0 wt % to about 15 wt %;
(c) Na2O in an amount ranging from about 9.0 wt % to about 20 wt %; and
(d) Li2O in an amount ranging from about 4.0 wt % to about 10 wt %.
In another embodiment, the present invention is directed to an alkali oxide-lime-silica glass composition suitable for immobilizing radioactive, hazardous, or mixed waste, comprising:
(a) SiO2 in an amount ranging from about 46 wt % to about 66 wt %;
(b) CaO in an amount ranging from about 5 wt % to about 28 wt %;
(c) Na2O in an amount ranging from about 1.9 wt % to about 25 wt %;
(d) Li2O in an amount ranging from about 3 wt % to about 11 wt %.
These compositions may contain an amount of B2O3 ranging from 0 wt % to less than 5 wt %.
In another embodiment, the present invention is directed to an alkali oxide-baria-silica glass composition suitable for immobilizing radioactive, hazardous, or mixed waste, comprising:
(a) SiO2 in an amount ranging from about 48 wt % to about 56 wt %;
(b) BaO in an amount ranging from about 3.5 wt % to about 7.0 wt %;
(c) Na2O in an amount ranging from about 8.0 wt % to about 15 wt %; and
(d) Li2O in an amount ranging from about 6.0 wt % to about 10.0 wt %.
These compositions may contain an amount of B2O3 ranging from 0 wt % to less than 5 wt %.
In another embodiment, the present invention is directed to an alkali oxide-magnesia-silica glass composition suitable for immobilizing radioactive, hazardous, or mixed waste, comprising:
(a) SiO2 in an amount ranging from about 40 wt % to about 68 wt %;
(b) MgO in an amount ranging from about 5.0 wt % to about 15 wt %;
(c) Na2O in an amount ranging from about 7.0 wt % to about 20 wt %; and
(d) Li2O in an amount ranging from about 3.0 wt % to about 9.0 wt %.
These compositions may contain an amount of B2O3 ranging from 0 wt % to less than 5 wt %.
The present invention can be more clearly understood from the following detailed description of specific embodiments thereof, which are not intended to limit the scope of the appended claims or of equivalents thereto.